The visual contents, whether these be fixed or moving images, are in general creations that benefit from guarantees of exclusivity associated with the creator's rights. Their reproduction is in general permitted only within a strictly defined framework that allows the creators and their beneficiaries to be remunerated.
To ensure that these legal rules are complied with correctly, many systems have been developed to prevent illegal copies or to make the quality of the copies sufficiently degraded to make them unusable.
Within this context, the patent application EP 1 237 369 aims to combat the copying of images by means of a camera while they are being displayed, for example using a camcorder in a movie theatre. In this document, it has been proposed to modulate temporally the amplitude of the brightness of selected pixels representing an anti-piracy message around the value to be displayed at a high rate that makes the message invisible to the human eye but generates artifacts in the sequence filmed by the camcorder. Such a solution requires a modulation at a rate higher than the flicker fusion frequency, which is of around 50 Hz, and therefore applies only to systems having a high image refresh rate, at least of around 100 Hz. Applied to systems with a lower display rate (50 Hz or 60 Hz for example), the modulation could be visible to the human eye and would visibly degrade the rendition of the displayed image.
The major drawback of this method is that the anti-piracy message can be easily removed by setting up the camcorder to low shutter speed settings.